1. Field of the Invention
The present invention relates to a delay measurement system, and in particular to a delay measurement system used for a distance measurement system in a mobile telephone system such as WCDMA (Wideband Code Division Multiple Access).
2. Description of the Related Art
In a prior art distance measurement or delay time measurement (hereinafter, simply referred to as delay measurement), an arrival timing of a delay measuring command outputted from a master device which serves as a measurement source and returned by a slave device has been measured. Thus, a shuttle time between the master device and the slave device has been measured, thereby obtaining a connection distance between both devices.
As for such prior art, there is a frame synchronization device of a mobile communication system, in which base stations performing wireless communication with mobile terminals by a time division multiple access method are connected over an IP network, provided with a transfer delay time calculating means transmitting a message to each base station with the IP network and statistically calculating a response time responding to the message as a transfer delay time for each base station using the IP network, a frame synchronous control means transmitting a frame synchronous message including transfer delay time information obtained by the transfer delay time calculating means for establishing a frame synchronization between the base stations, and a frame timing generation means receiving the frame synchronous message from the frame synchronous control means and generating a frame timing by the base station based on frame synchronous information (see e.g. patent document 1).
[Patent document 1]
Japanese Patent Application Laid-open No. 2000-324535
In the above-mentioned prior art, a timing of receiving a return signal for delay measurement at a master device fluctuates according to a distance between the master device and a slave device, thereby providing an arbitrary timing.
Therefore, at the timing of receiving the return signal for delay measurement in the master device, a signal of main information or the like can not be received in order to avoid collision. Also, a connection distance between the master device and the slave device is not known at the initial time of a connection. Therefore, an estimated reception timing of the above-mentioned delay measuring return signal requires a delay measuring window width to some extent, resulting in a problem that a transmission capacity for the main information is reduced. Also, there is a problem that the longer the connection distance becomes and the higher the transmission speed becomes, the larger the ratio of the transmission capacity occupied by the delay measuring window width becomes.
Also, when a delay time is measured by assigning a delay measuring signal to free bytes of an overhead, a delay time specific to each device is not known, so that an accurate delay time can not be measured.
Also, in a system arrangement where a plurality of slave devices are connected to the master device in cascade, when the distance from the master device to each slave device is measured and when a transmission signal is regenerated and relayed between the slave devices, the regeneration and relay of the delay measuring signal are also required. However, when the timing of the delay measuring signal overlaps a timing of an overhead portion that is a fixed timing of a transmission frame, a phase fluctuation occurs at the timing of the delay measuring signal since a transfer is performed avoiding the overhead portion. Accordingly, if the phase fluctuation is accumulated by the cascade connection, such a problem that a delay measurement error becomes large has occurred.